This description relates to a network system and, more particularly, to a network system forming a network by connecting a plurality of control systems to a ring topology, and enabling the control systems to transmit data in both directions so that the control systems can transmit data to each other.
In a control field requiring an organic operation in a plurality of control systems, a communication among the control systems requires a high reliability. The reliability should be secured by performing a proper operation when the communication in the control systems fails. When occurrence of a sudden unstable power supply from outside or communication failure due to malfunction of communication media continues, all the control systems connected to a network system cannot transmit or receive a predetermined data within a predetermined time. The control systems requiring a mutual organic operation are prevented from operating normally if the communication is disabled. The network system involving frequent communication failures cannot be used in the control field.
In a recently-developed control system, a large amount of data is exchanged between PLCs (Programmable Logic Controllers), PLCs and input/output apparatus, and PLCs and PCs (Personal Computers) using an Ethernet methodology, being different from that of the past where input and output functions were major functions. That is, the control systems employing the Ethernet network quickly exchanges data therebetween.
As a result, it is imperative that the control systems improve reliability of the Ethernet network, and overall reliability of the control systems cannot be guaranteed unless reliability of the Ethernet network is obtained. Efforts are being waged for securing the reliability of the entire Ethernet network-based control systems by acquiring and guaranteeing the reliability of the Ethernet-based network in the control systems.
The Ethernet, as one of the in-home private communication-networks, is a local network system where up to 1024 data stations can exchange information with a 10 Mbps of transmission speed within a distance about 2.5 km. Further, transmission media that transmits data can be largely categorized into three types based on transmission data, that is, a 50 Ω coaxial cable (10BASE2, 10BASE5), a twisted pair including a twisted pair cable (10BASE-T), and an optical fiber cable (100BASE-FX).
A maximum bandwidth of the coaxial cable is 10 Mbps, and the maximum transmission distance is 185 m for the 10BASE2 and 500 m for the 10BASE5. The maximum bandwidth of the twisted pair of 10BASE-T is 10 Mbps, and a maximum transmission distance is 100 m. A maximum bandwidth of the optical fiber cable of 100BASE-FX is 100 Mbps, and a maximum transmission distance is 2 km for a multiple mode and several tens of meters for a single mode.
In configuring a network system with the Ethernet, a network system using hubs and having a phase structure of star topology is commonly used. The network system having the phase structure of star topology is composed of a radial shape phase structure having one topology, and a plurality of control systems is connected by the twisted pair cable in one hub.
A communication failure may be generated when the power supplied to a hub being a center of the network system having a phase structure of star topology is unstable or when a hub is operated unstably due to malfunctions. The communication failure disturbs a data transmission in each of the plurality of the control systems. Therefore, it is unsuitable for the network system having the phase structure of star topology to be used in the control field that requires a high reliability.
The network system using a plurality of hubs and having a phase structure of a combination of a star topology and a ring topology is well known. The network system using the plurality of the hubs and having the combined phase structure of star topology and ring topology is such that the plurality of the hubs is connected by the star topology and the ring topology and each of the hubs is connected to the plurality of the control systems by the star topology.
The mutual communication between the plurality of control systems connected to the hubs that operate normally can be performed, even though one of the hubs malfunctions in the network system having the combined phase structure of star topology and ring topology. Therefore, the control systems connected to the hubs that operate normally can perform a normal communication thereamong, even if one of the hubs malfunctions.
However, there is a drawback in the network system having the combined phase structure of star topology and ring topology thus described in that it requires additional plural hubs, and a length of a transmission medium connecting the control systems to the hubs tends to elongate.